The position of a piston, such as a valve or actuator body, may be affected by controlling the pressures of fluids applied to one or both ends thereof. These pressures may be controlled in turn by a device such as an electrohydraulic servovalve (EHSV) which uses a first stage motor to control the position of a second stage spool. Moving the spool opens and closes various fluid passages to control the pressure sources connected to the valve or actuator body. The present invention will be described herein in terms of a valve or actuator body that is positioned by the pressures of fuel in an aircraft fuel system, it being understood that it is not limited to use in such systems. The general term “piston” is used herein to describe a structure that could be either a valve, such as a fuel metering valve, or an actuator body, such as may be used for positioning variable geometry guide vanes (or moving other parts) on an aircraft.
The piston positioned by the control system may be a fuel metering valve, the position of which affects fuel flow to a gas turbine engine. Alternately, the piston may be connected to movable elements, such as variable geometry guide vanes, the position of which is controlled by the position of the piston. In the event of a power interruption to the control system, it may be desirable to maintain the piston in position until power is restored. However, when power is interrupted, system pressures drop and the fluid holding the piston in position may drain and allow the piston to deviate from the position it was in when the power interruption occurred. While it is known to shift an EHSV to a failsafe position in the event of a power interruption, EHSV second stage spools leak and cannot adequately prevent fluid flows or hold a piston in position.
This problem has been addressed by the inclusion of FFV's between the EHSV and the piston, but the control of these valves has heretofore been complicated. Furthermore, known FFV's sometimes allow the position of a piston to shift before finally coming to rest in a fixed position. It would therefore be desirable to provide a fuel system having a piston controlled by the position of a spool, such as a second stage spool of an EHSV, and an FFV for selectively fixing the piston in position which is simple in construction and in which, optionally, the position of the piston may be substantially maintained in the position it occupied at a power loss.